The S region of the murine major histocompatibility complex (H-2) was defined twenty years ago as a genetic locus controlling the quantity of the Ss (serum substance) protein in mouse serum. Since then, work from a number of laboratories has established that Ss is composed of two distinct proteins: C4, the murine fourth component of complement, and Slp (for sex-limited protein), a protein which shares extensive structural and biochemical homology with C4 but which lacks complement activity. Slp is distinctive in that, for some inbred mouse strains, its expression depends entirely or almost entirely on testosterone levels, while for other strains its synthesis is either independent of testosterone levels, or undetectable. Genetic analysis indicates that the S region harbors distinct structural genes for C4 and Slp. The goal of the proposed program is to use nuclei acid cloning and sequencing methods (1) to establish the structures of the C4 and Slp molecules, (2) to determine the relative locations of their respective genes in the H-2 complex, and (3) to examine the nature of nucleic acid structural variations which impart altered levels of protein functional activity and protein expression. The structural information we accumulate should provide insights into the evolution of complement and of the murine histocompatibility complex, the regulation of C4 and Slp expression, and the structure/function relationships among C4, Slp, and two closely related complement components, C3 and C5.